Recent electronic devices deal with the increased amount of transmitted information by employing an optical interconnect in addition to electric wiring, and opto-electric hybrid boards capable of transmitting an electric signal and an optical signal at the same time are used in many cases. An example of known opto-electric hybrid boards has the structure illustrated in FIG. 17, where an insulating layer 1 made of polyimide or the like serves as a substrate. Electric wiring 2, which is a conductive pattern, is provided on a front surface of the insulating layer 1, thereby forming an electric circuit board E. An optical waveguide W is provided on the rear side of the electric circuit board E to be optically coupled to an optical device that is installed at a given point in the electric wiring 2. The front surface of the electric circuit board E is insulated and protected by a coverlay 3. The optical waveguide W is made up of three layers: an under cladding layer 6, a core 7 serving as a path along which light travels, and an over cladding layer 8.
Applications of the opto-electric hybrid board described above (an opto-electric hybrid board 10) include installing the opto-electric hybrid board 10 in an electronic device as it is, and using the opto-electric hybrid board 10 that is formed into a ribbon shape and that has a photoelectric connection ferrule mounted to a front end of the belt as a connector configured to connect one board to another board or one chip to another chip on a board.
As illustrated in FIG. 17, the opto-electric hybrid board 10 in general is shaped so that left and right edge portions of the electric circuit board E (portions surrounded by dot-dash lines X in FIG. 17) protrude farther outward than left and right edge portions of the optical waveguide W on both sides along the longitudinal direction of the optical waveguide W. This is due to how the opto-electric hybrid board 10 is usually manufactured, that is, the electric circuit board E is created first and then the under cladding layer 6, the core 7, and the over cladding layer 8 are formed and stacked in the order stated on a rear surface of the electric circuit board E (namely, the rear surface of the insulating layer 1 made of polyimide or other materials) in a given pattern by photolithography or other methods. It is a common technical practice to form the optical waveguide W so that the outline of the optical waveguide W is contained inside the rear surface shape of the electric circuit board E because the pattern is formed by removing unnecessary portions after flat layers are formed. This is also practiced in some of general optical waveguides proposed, and such optical waveguides are formed to have a size smaller than a substrate having one side on which the optical waveguides are formed (see JP-A-2014-115480).